The sock_diag netlink subsystem provides a mechanism for obtaining
information about sockets of various address families from the
kernel. This subsystem can be used to obtain information about
individual sockets or request a list of sockets.
In the request, the caller can specify additional information it
would like to obtain about the socket, for example, memory
information or information specific to the address family.
When requesting a list of sockets, the caller can specify filters
that would be applied by the kernel to select a subset of sockets to
report. For now, there is only the ability to filter sockets by
state (connected, listening, and so on.)
Note that sock_diag reports only those sockets that have a name; that
is, either sockets bound explicitly with bind(2) or sockets that were
automatically bound to an address (e.g., by connect(2)). This is the
same set of sockets that is available via /proc/net/unix,
/proc/net/tcp, /proc/net/udp, and so on.
Request
The request starts with a struct nlmsghdr header described in
netlink(7) with nlmsg_type field set to SOCK_DIAG_BY_FAMILY. It is
followed by a header specific to the address family that starts with
a common part shared by all address families:
struct sock_diag_req {
__u8 sdiag_family;
__u8 sdiag_protocol;
};
The fields of this structure are as follows:
sdiag_family
An address family. It should be set to the appropriate AF_*
constant.
sdiag_protocol
Depends on sdiag_family. It should be set to the appropriate
IPPROTO_* constant for AF_INET and AF_INET6, and to 0 other‐
wise.
If the nlmsg_flags field of the struct nlmsghdr header has the
NLM_F_DUMP flag set, it means that a list of sockets is being
requested; otherwise it is a query about an individual socket.
Response
The response starts with a struct nlmsghdr header and is followed by
an array of objects specific to the address family. The array is to
be accessed with the standard NLMSG_* macros from the netlink(3) API.
Each object is the NLA (netlink attributes) list that is to be
accessed with the RTA_* macros from rtnetlink(3) API.
UNIX domain sockets
For UNIX domain sockets the request is represented in the following
structure:
struct unix_diag_req {
__u8 sdiag_family;
__u8 sdiag_protocol;
__u16 pad;
__u32 udiag_states;
__u32 udiag_ino;
__u32 udiag_show;
__u32 udiag_cookie[2];
};
The fields of this structure are as follows:
sdiag_family
The address family; it should be set to AF_UNIX.
sdiag_protocolpad These fields should be set to 0.
udiag_states
This is a bit mask that defines a filter of sockets states.
Only those sockets whose states are in this mask will be
reported. Ignored when querying for an individual socket.
Supported values are:
1 << TCP_ESTABLISHED
1 << TCP_LISTENudiag_ino
This is an inode number when querying for an individual
socket. Ignored when querying for a list of sockets.
udiag_show
This is a set of flags defining what kind of information to
report. Each requested kind of information is reported back
as a netlink attribute as described below:
UDIAG_SHOW_NAME
The attribute reported in answer to this request is
UNIX_DIAG_NAME. The payload associated with this
attribute is the pathname to which the socket was bound
(a sequence of bytes up to UNIX_PATH_MAX length).
UDIAG_SHOW_VFS
The attribute reported in answer to this request is
UNIX_DIAG_VFS. The payload associated with this
attribute is represented in the following structure:
struct unix_diag_vfs {
__u32 udiag_vfs_dev;
__u32 udiag_vfs_ino;
};
The fields of this structure are as follows:
udiag_vfs_dev
The device number of the corresponding on-disk
socket inode.
udiag_vfs_ino
The inode number of the corresponding on-disk
socket inode.
UDIAG_SHOW_PEER
The attribute reported in answer to this request is
UNIX_DIAG_PEER. The payload associated with this
attribute is a __u32 value which is the peer's inode
number. This attribute is reported for connected sock‐
ets only.
UDIAG_SHOW_ICONS
The attribute reported in answer to this request is
UNIX_DIAG_ICONS. The payload associated with this
attribute is an array of __u32 values which are inode
numbers of sockets that has passed the connect(2) call,
but hasn't been processed with accept(2) yet. This
attribute is reported for listening sockets only.
UDIAG_SHOW_RQLEN
The attribute reported in answer to this request is
UNIX_DIAG_RQLEN. The payload associated with this
attribute is represented in the following structure:
struct unix_diag_rqlen {
__u32 udiag_rqueue;
__u32 udiag_wqueue;
};
The fields of this structure are as follows:
udiag_rqueue
For listening sockets: the number of pending
connections. The length of the array associated
with the UNIX_DIAG_ICONS response attribute is
equal to this value.
For established sockets: the amount of data in
incoming queue.
udiag_wqueue
For listening sockets: the backlog length which
equals to the value passed as the second argu‐
ment to listen(2).
For established sockets: the amount of memory
available for sending.
UDIAG_SHOW_MEMINFO
The attribute reported in answer to this request is
UNIX_DIAG_MEMINFO. The payload associated with this
attribute is an array of __u32 values described below
in the subsection "Socket memory information".
The following attributes are reported back without any spe‐
cific request:
UNIX_DIAG_SHUTDOWN
The payload associated with this attribute is __u8
value which represents bits of shutdown(2) state.
udiag_cookie
This is an array of opaque identifiers that could be used
along with udiag_ino to specify an individual socket. It is
ignored when querying for a list of sockets, as well as when
all its elements are set to -1.
The response to a query for UNIX domain sockets is represented as an
array of
struct unix_diag_msg {
__u8 udiag_family;
__u8 udiag_type;
__u8 udiag_state;
__u8 pad;
__u32 udiag_ino;
__u32 udiag_cookie[2];
};
followed by netlink attributes.
The fields of this structure are as follows:
udiag_family
This field has the same meaning as in struct unix_diag_req.
udiag_type
This is set to one of SOCK_PACKET, SOCK_STREAM, or SOCK_SEQ‐PACKET.
udiag_state
This is set to one of TCP_LISTEN or TCP_ESTABLISHED.
pad This field is set to 0.
udiag_ino
This is the socket inode number.
udiag_cookie
This is an array of opaque identifiers that could be used in
subsequent queries.
IPv4 and IPv6 sockets
For IPv4 and IPv6 sockets, the request is represented in the follow‐
ing structure:
struct inet_diag_req_v2 {
__u8 sdiag_family;
__u8 sdiag_protocol;
__u8 idiag_ext;
__u8 pad;
__u32 idiag_states;
struct inet_diag_sockid id;
};
where struct inet_diag_sockid is defined as follows:
struct inet_diag_sockid {
__be16 idiag_sport;
__be16 idiag_dport;
__be32 idiag_src[4];
__be32 idiag_dst[4];
__u32 idiag_if;
__u32 idiag_cookie[2];
};
The fields of struct inet_diag_req_v2 are as follows:
sdiag_family
This should be set to either AF_INET or AF_INET6 for IPv4 or
IPv6 sockets respectively.
sdiag_protocol
This should be set to one of IPPROTO_TCP, IPPROTO_UDP, or
IPPROTO_UDPLITE.
idiag_ext
This is a set of flags defining what kind of extended informa‐
tion to report. Each requested kind of information is
reported back as a netlink attribute as described below:
INET_DIAG_TOS
The payload associated with this attribute is a __u8
value which is the TOS of the socket.
INET_DIAG_TCLASS
The payload associated with this attribute is a __u8
value which is the TClass of the socket. IPv6 sockets
only. For LISTEN and CLOSE sockets, this is followed
by INET_DIAG_SKV6ONLY attribute with associated __u8
payload value meaning whether the socket is IPv6-only
or not.
INET_DIAG_MEMINFO
The payload associated with this attribute is repre‐
sented in the following structure:
struct inet_diag_meminfo {
__u32 idiag_rmem;
__u32 idiag_wmem;
__u32 idiag_fmem;
__u32 idiag_tmem;
};
The fields of this structure are as follows:
idiag_rmem The amount of data in the receive queue.
idiag_wmem The amount of data that is queued by TCP
but not yet sent.
idiag_fmem The amount of memory scheduled for future
use (TCP only).
idiag_tmem The amount of data in send queue.
INET_DIAG_SKMEMINFO
The payload associated with this attribute is an array
of __u32 values described below in the subsection
"Socket memory information".
INET_DIAG_INFO
The payload associated with this attribute is specific
to the address family. For TCP sockets, it is an
object of type struct tcp_info.
INET_DIAG_CONG
The payload associated with this attribute is a string
that describes the congestion control algorithm used.
For TCP sockets only.
pad This should be set to 0.
idiag_states
This is a bit mask that defines a filter of socket states.
Only those sockets whose states are in this mask will be
reported. Ignored when querying for an individual socket.
id This is a socket ID object that is used in dump requests, in
queries about individual sockets, and is reported back in each
response. Unlike UNIX domain sockets, IPv4 and IPv6 sockets
are identified using addresses and ports. All values are in
network byte order.
The fields of struct inet_diag_sockid are as follows:
idiag_sport
The source port.
idiag_dport
The destination port.
idiag_src
The source address.
idiag_dst
The destination address.
idiag_if
The interface number the socket is bound to.
idiag_cookie
This is an array of opaque identifiers that could be used
along with other fields of this structure to specify an indi‐
vidual socket. It is ignored when querying for a list of
sockets, as well as when all its elements are set to -1.
The response to a query for IPv4 or IPv6 sockets is represented as an
array of
struct inet_diag_msg {
__u8 idiag_family;
__u8 idiag_state;
__u8 idiag_timer;
__u8 idiag_retrans;
struct inet_diag_sockid id;
__u32 idiag_expires;
__u32 idiag_rqueue;
__u32 idiag_wqueue;
__u32 idiag_uid;
__u32 idiag_inode;
};
followed by netlink attributes.
The fields of this structure are as follows:
idiag_family
This is the same field as in struct inet_diag_req_v2.
idiag_state
This denotes socket state as in struct inet_diag_req_v2.
idiag_timer
For TCP sockets, this field describes the type of timer that
is currently active for the socket. It is set to one of the
following constants:
0 no timer is active
1 a retransmit timer
2 a keep-alive timer
3 a TIME_WAIT timer
4 a zero window probe timer
For non-TCP sockets, this field is set to 0.
idiag_retrans
For idiag_timer values 1, 2, and 4, this field contains the
number of retransmits. For other idiag_timer values, this
field is set to 0.
idiag_expires
For TCP sockets that have an active timer, this field
describes its expiration time in milliseconds. For other
sockets, this field is set to 0.
idiag_rqueue
For listening sockets: the number of pending connections.
For other sockets: the amount of data in the incoming queue.
idiag_wqueue
For listening sockets: the backlog length.
For other sockets: the amount of memory available for sending.
idiag_uid
This is the socket owner UID.
idiag_inode
This is the socket inode number.
Socket memory information
The payload associated with UNIX_DIAG_MEMINFO and INET_DIAG_SKMEMINFO
netlink attributes is an array of the following __u32 values:
SK_MEMINFO_RMEM_ALLOC
The amount of data in receive queue.
SK_MEMINFO_RCVBUF
The receive socket buffer as set by SO_RCVBUF.
SK_MEMINFO_WMEM_ALLOC
The amount of data in send queue.
SK_MEMINFO_SNDBUF
The send socket buffer as set by SO_SNDBUF.
SK_MEMINFO_FWD_ALLOC
The amount of memory scheduled for future use (TCP only).
SK_MEMINFO_WMEM_QUEUED
The amount of data queued by TCP, but not yet sent.
SK_MEMINFO_OPTMEM
The amount of memory allocated for the socket's service needs
(e.g., socket filter).
SK_MEMINFO_BACKLOG
The amount of packets in the backlog (not yet processed).

NETLINK_INET_DIAG was introduced in Linux 2.6.14 and supported
AF_INET and AF_INET6 sockets only. In Linux 3.3, it was renamed to
NETLINK_SOCK_DIAG and extended to support AF_UNIX sockets.
UNIX_DIAG_MEMINFO and INET_DIAG_SKMEMINFO were introduced in Linux
3.6.

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2019-03-06 SOCK_DIAG(7)